Tuberculosis (TB) is one of the major global health issue, and a serious health concern in in many countries, especially in countries where the prevalence and spread of multidrug-resistant tuberculosis (MDR-TB) and extensively drug resistant TB (XDR-TB) has increased during the last few years. Early diagnosis of the disease and the rapid identification of resistance to primary anti-TB drugs are essential for efficient treatment, prevention and control of TB.
The diagnosis of tuberculosis in many countries still relies on tuberculin skin test (TST) and direct sputum examination by light microscopy. TST has low specificity due to cross-reactivity to protein purified derivative (PPD) antigens shared by environmental mycobacteria species, and may give false positive responses in Bacillus Calmette-Guerin (BCG) vaccinated individuals, particularly those who received multiple BCG vaccinations. BCG policies vary considerably between countries, primarily depending on the current epidemiological situation.
Interferon-γ release assays (IGRA) have been designed to overcome the problem of cross-reactive T cell immune responses by measuring immune responses to antigens specific for M. tuberculosis. Neither the TST nor the IGRA however is able to discriminate between active TB-disease, latent TB infection and previous TB-infection. Exposure to Mycobacteria other than tuberculosis may lead to false-positive results, and poor specificity of the tests may lead to unnecessary prophylactic treatment with anti-tuberculosis drugs. Thus, the ideal diagnostic test should not only discriminate latent TB infection from active TB, but also discriminate between TB, exposure to other Mycobacteria and previous BCG vaccination.
Although latent TB is clinically silent and not contagious, it can reactivate to cause contagious pulmonary TB (Flynn and Chan, Tuberculosis: latency and reactivation. Infect Immun, 2001. 69(7): p. 4195-201). Tubercle bacilli are generally considered to be non-replicating in latent TB, yet it may slowly grow and replicate (Munoz-Elias et al., Replication dynamics of Mycobacterium tuberculosis in chronically infected mice. Infect Immun, 2005. 73(1): p. 546-51). Latent TB is characterized by highly reduced metabolism and a significantly altered gene expression (Voskuil et al., Inhibition of respiration by nitric oxide induces a Mycobacterium tuberculosis dormancy program. J Exp Med, 2003. 198(5): p. 705-13) associated with the different stages of infection (Lin, M. Y. and T. H. Ottenhoff, Not to wake a sleeping giant: new insights into host-pathogen interactions identify new targets for vaccination against latent Mycobacterium tuberculosis infection. Biol Chem, 2008. 389(5): p. 497-511). Since most cases of active TB arise in people with latent TB, there is an urgent need to identify new potential targets for TB diagnosis and for development of new and better TB vaccines. Different vaccine targets may be useful for countries with high versus countries with low M. tuberculosis burden; or for individuals who have not yet been exposed versus exposed individuals. Thus, a possible solution is the provision of different M. tuberculosis vaccines developed, depending on natural M. tuberculosis exposures of the target population.
One of the strategies in developing new diagnostic methods and in improving the TB vaccine involves the identification of epitopes in antigens that induce T cell responses. In, addition a strategic choice of the vaccine candidate may be as important, i.e. to choose i) M. tuberculosis antigens which are rather not secreted (these may rather serve as ‘decoy antigens which prevents the immune system to focus on the biologically and clinically relevant targets), ii) antigens that play a role in M. tuberculosis pathogenicity and iii) antigens that are expressed at different points of the M. tuberculosis infection and the M. tuberculosis life cycle. The antigens according to the present invention fulfill these criteria.
CD4+ T cells play a central role in M. tuberculosis—directed cellular immune responses (Endsley et al. Mycobacterium bovis BCG vaccination induces memory CD4 T cells characterized by effector biomarker expression and antimycobacterial activity. Vaccine 2007. 25:8384-8394.). It is most likely that an effective tuberculosis (TB) vaccine would target the expansion of CD8+ and CD4+ T cells, which recognize M. tuberculosis peptides presented by major histocompatibility complex (MHC) class I and class II molecules. The MHC locus is the most variable gene locus in the human genome, and the variability of MHC class II alleles in different populations is well documented Certain MHC class II alleles have been shown to be associated with M. tuberculosis infection (Kettaneh et al. Human leukocyte antigens and susceptibility to tuberculosis: a metaanalysis of case-control studies. Int. J. Tuberc. Lung Dis. 2006. 10:717-725). DRB1*0803 and DQB1*0601 were found to be associated with TB disease progression, development of drug resistance, and disease severity in Koreans (Kim et al. Association of HLA-DR and HLA-DQ genes with susceptibility to pulmonary tuberculosis in Koreans: preliminary evidence of associations with drug resistance, disease severity, and disease recurrence. Hum. Immunol. 2005. 66:1074-1081). In South Africa, DRB1*1302 and DQB1*0301 to −0304 were apparently associated with active TB compared to control individuals lacking these alleles (Lombard et al. Association of HLA-DR, -DQ, and vitamin D receptor alleles and haplotypes with tuberculosis in the Venda of South Africa. Hum. Immunol. 2006. 67:643-654). The prevalence of HLA DRB1* 0401 and HLA-DRB1*0801 was significantly decreased in Mexican patients with pulmonary TB compared to their prevalence in healthy controls (Teran-Escandon et al. Human leukocyte antigen-associated susceptibility to pulmonary tuberculosis: molecular analysis of class II alleles by DNA amplification and oligonucleotide hybridization in Mexican patients. Chest 1999. 115:428-433). The association of some MHC class II alleles with “better disease outcome” could be due to the fact that these alleles are “better” at binding and presenting a certain repertoire of peptide epitopes to CD4+ T cells than other alleles Not mutually exclusive, the quality (i.e. differential cytokine production, cytotoxic responses, of a cellular immune response) and the quantity of a cellular response is also biologically and clinically relevant. In addition, it will be of great value where M. tuberculosis antigen-specific T-cells reside, i.e. in terminally different effector T-cells (they may produce potent anti-M. tuberculosis directed activities, yet they may be short-lived), in memory immune cells or in precursor T-cells which will be able to replenish the T-cell pool once immune memory T-cells and terminally differentiated effector T-cells are not available anymore, e.g. they have succumbed to activation-induced cell death after repetitive antigen-exposure. The preferential expansion of anti-M. tuberculosis directed immune responses in different immune T-cell subsets, particularly in precursor T-cells, will therefore be advantageous.
It is not only important where M. tuberculosis specific T-cells reside (precursor, immune memory of terminally differentiated T-cells), yet also the nature of the target antigens. The identification of peptides binding to molecularly defined MHC class II alleles could therefore represent an important first step in identifying potential targets for TB vaccine design and the development of new diagnostic assays. More recently, De Groot and colleagues used a bioinformatics approach, followed by validation with functional assays to identify CD4+ T-cell epitopes that were used to construct an epitope-based M. tuberculosis vaccine (Developing an epitope-driven tuberculosis (TB) vaccine. Vaccine 2005. 23:2121-2131). Only a few M. tuberculosis MHC class II binding peptides have been identified so far, and 7% of the M. tuberculosis open reading frames have been explored for both B-cell and T-cell epitopes (Blythe et al. An analysis of the epitope knowledge related to mycobacteria. Immunome Res. 2007. 3:10). Peptide microarray assay has the major advantage that a high number of candidate peptides can be screened within a short time frame. Gaseitsiwe et al. (Peptide microarray-based identification of Mycobacterium tuberculosis epitope binding to HLA-DRB1*0101, DRB1*1501 and DRB1*0401. Clin Vacc Immunol 2010, 17:168-175) describe M. tuberculosis peptide binding to the three most frequently encountered MHC class II alleles in different populations; DRB1*0101, DRB1*1501, and DRB1*0401. DRB1*0101, DRB1*1501, and DRB1*0401 exhibit population frequencies of 15.4%, 32.9%, and 20.9% among Caucasians. In the Botswana population, HLA-DRB1*01, -DRB1*02, and -DRB1*04 show population frequencies of 21.7%, 21.3%, and 14.4%, respectively. 7,446 unique peptides derived from 61 M. tuberculosis proteins were tested. In addition, M. tuberculosis targets can be recognized by serum antibodies. This is not only relevant concerning immunoglobulin-mediated recognition, it is also an indirect measurement of T-cell activation and recognition, since strong IgG-responses require strong CD4+ T-cell help (Gaseitsiwe et al. supra; Gaseitsiwe et al. Pattern recognition in pulmonary tuberculosis defined by high content peptide microarray chip analysis representing 61 proteins from M. tuberculosis. PLoS ONE. 2008; 3(12):e3840. Epub 2008 Dec. 9).